Abstract

In order to remove motion artefact prior to digital processing, the invention provides a method of monitoring living tissue comprising the steps of emitting electromagnetic radiation at said tissue at at least first and second different wavelengths, receiving the radiation at the different wavelengths after it has been transmitted through or reflected within said tissue, providing at least first and second signals which are a logarithmic measure of the received first and second radiation wavelengths and subtracting the second signal from the first signal, removing a DC component of the result of the subtraction and providing an AC component to digital sampling means, and processing the digital samples in order to provide a desired value representing a property of the tissue.

Description

ARTEFACT REDUCTION IN PHOTOPLETHYSMOGRAPHY

The present invention concerns the reduction of artefact in photoplethysmography, in particular the reduction of movement artefact. It is particularly significant in the context of remote (i.e. non-contact) photoplethysmography and pulse oximetry.

Photoplethysmography (PPG) is a well known technique for monitoring non- invasively variations in the light absorption by a component being monitored (e.g. oxygen in the blood stream). The major application for PPG at the present time is in pulse oximetry, used for non-invasive oxygen saturation measurement. The majority of pulse oximeters utilise PPG signals obtained by light transmission through tissue, rather than by reflection. The measurement of arterial oxygen saturation is made by obtaining and quantitatively comparing at least two PPG signals at different optical wavelengths (usually visible red and infra-red) from which an indication of blood colour (and hence oxygenation) is made. The technique requires the measurement of arterial pulsations, since these are used in a calibration process. Corruption of the PPG signal arises from inadvertent measurement of ambient light (ambient artefact) and from voluntary and involuntary subject movement (movement artefact).

Ambient artefact can, in theory, be removed in the recovery of photoplethysmographic signals, provided that the instrumentation correctly subtracts the ambient signal and that appropriate sensor packaging is utilised. Recent clinical publications however suggest that ambient light is a problem in practical systems, mainly due to the severity of artefact experienced in the operating theatre. It is not uncommon practice for clinicians to cover the finger and pulse oximeter with a black bag to eliminate problems caused by particularly strong ambient lighting.

Movement artefact is by far the most important problem with current technology. An ability to significantly reduce or even remove the effect of such artefact would be of great importance in patient monitoring, particularly in the case of patients who have been difficult to monitor in the past. Movement artefact is manifested by mechanical forces giving rise to changes in:

All PPG systems respond to changes in venous blood volume as well as arterial blood volume changes, indeed this is used clinically in the study of venous insufficiency, for example. However changes in venous blood volume may be caused variously by:

1. physiology (e.g. exercise or temperature)

2. gravity (e.g. raising limb or bending down)

3. pressure (e.g. pressure on the probe or pressure on the body) 4. other mechanical forces (e.g. stress, strain)

As long as these changes take place on time scales greater than a few heart cycles, the conventional implementation of pulse oximetry will (in theory) be unaffected, as the method is insensitive to quasi-static changes in the non-pulsatile tissue characteristics. PPG studies in contexts other than oximetry relate to both arterial and venous blood changes, and there is therefore a clear distinction to be drawn between the nature of artefact in the context of pulse oximetry (i.e. all dynamic components which do not originate from the arterial blood pulsations) and that of PPG (i.e. all dynamic components which do not originate from changes in blood volume).

If the consequences of a typical mechanical movement which might give rise to severe artefact are considered, such as a subject attached to a finger clip raising their arm to take hold of an object, then it is likely that all the above mentioned types of artefact will be generated. The physical contact of the probe to the skin is a major factor in generating secondary artefacts. In taking note of these effects, pulse oximeter designers have made more significant progress in improving the mechanical design of finger clips, ear clips and more recently semi-disposable adhesive probes. This progress has proceeded in parallel with other efforts to eliminate the undesirable effects of artefact, most notably by signal processing methods.

There have been a number of attempts at tackling the problem of artefact in the context of pulse oximetry, for example: 1. Correlation cancellation (for example, as described in International

Both 1 and 3 ideally use an independent measurement of artefact supplied by another transducer (e.g. piezo or optical). Both 1 and 3 assume that artefact is a linear addition to the pulsatile signal. Basic observation of the PPG signal under typical artefact-producing conditions casts doubt on this hypothesis. 2 provides a method of output suppression in the presence of artefact, established by a simple feature recognition analysis. However this approach is not a solution to artefact equalisation. In 4, the processing of the PPG signals is carried out following digital conversion. There is no attempt to eliminate the motion signal in any way, by measuring the motion signal, cancelling it by feedback, or mathematical manipulation.

Rather the digital processing uses assumptions based on characteristics of the motion signal. 4 assumes a non-linear (multiplicative) model for the motion artefact and a Beer-Lambert law for the light transmission through tissue. The artefact becomes additive following a logarithmic transformation of the PPG signals, an expression for the oxygen saturation is derived by means of a linear solution of the three equations derived from observations of PPG signals at three source wavelengths. The calculation of oxygen saturation is unnecessarily complex, and any practical implementation would require an unnecessarily complex calibration involving the determination of at least six coefficients (page 11 of the published patent application).

In 5, US-A-4,714,341, periods of artefact, are identified and the oximeter output is subsequently suppressed, rather than remaining the artefact from the PPG signals. A third light source with a different wavelength is included, so that the oxygen saturation may be calculated simultaneously with the red and third wavelength, and the infrared and third wavelength. Assuming that artefact affects the signals in a similar manner,
the two values for the calculated saturation will be different during periods of artefact and the result can be disregarded.

The present invention seeks to approach the problem of artefact from a new direction based on observations made by the inventors of the nature of typical artefact and on a physical model for the light/tissue interaction in the presence of artefact. A key element of the model is the optical receiver, which has a non-linear response characteristic. This property of the receiver is important in providing calibration of the signals and leads to a very effective method for artefact equalisation. The method may be implemented by analogue electronics and is suitable for electronic miniaturisation. In the present invention, the optical receiver is modelled, employing the following assumptions:

1. The received light has additive components due to direct coupling with the light source, coupling through non-pulsatile tissue and pulsatile tissue.

2. The light travelling through non-pulsatile tissue and pulsatile tissue is modulated by movement artefact and this artefact is represented by a multiplicative coefficient.

3. The light receiver provides a logarithmic response, either in the light receiving diode itself, or in a subsequent logarithmic amplifier, and two such light signals at different wavelengths are subtracted from one another. This it will be shown provides a large DC signal which is of no interest and can be filtered out and a smaller AC component which represents the light signal component due to pulsatile tissue, but with the motion artefact removed.

4. The AC component signal representing light through pulsatile tissue may then be digitised and subjected to DSP operations in order to derive values representing oxygen saturation.

Accordingly, in a first aspect the present invention provides a method of monitoring living tissue comprising the steps of emitting electromagnetic radiation at said tissue at at least first and second different wavelengths, receiving the radiation at the different wavelengths after it has been transmitted through or reflected within said tissue, providing at least first and second signals which are a logarithmic measure of the received first and second radiation wavelengths and subtracting the second signal from
the first signal, removing a DC component of the result of the subtraction and providing an AC component to digital sampling means, and processing the digital samples in order to provide a desired value representing a property of the tissue.

In a preferred application of the invention to pulse oximetry a third wavelength of electromagnetic radiation is employed and the second wavelength signal is subtracted from a logarithmic form of the signal produced in response to the third wavelength in order to provide a further AC component resulting from this subtraction which is applied to the digital sampling needs, thereby oxygen saturation may be calculated. In a further aspect, there is provided apparatus for measuring or monitoring living tissue, comprising means for irradiating tissue with electromagnetic radiation at at least first and second different wavelengths, means for receiving the radiation at the different wavelengths after it has been transmitted through or reflected within said tissue for providing respective first and second wavelength signals which are a logarithmic measure of the received radiation signals, means for subtracting the second wavelength signal from the first wavelength signal, means for removing a DC component from the output of the subtracting means, digital sampling means for digitising said AC component, and processing means for processing the digital samples in order to derive a value representing a property of said tissue. In a still further aspect, there is provided apparatus for measuring or monitoring living tissue, comprising means for irradiating tissue with electromagnetic radiation at at least first and second different wavelengths, means for receiving the radiation at the different wavelengths after it has been transmitted through or reflected within said tissue, logarithmic amplifier means coupled to the receiving means for providing respective first and second wavelength signals which are a logarithmic measure of the received radiation signals, means for subtracting the second wavelength signal from the first wavelength signal, means for removing a DC component from the output of the subtracting means, and processing means for processing the digital samples in order to derive a value representing a property of said tissue. In accordance with the invention, motion artefact can be completely or at least very substantially eliminated prior to any digital processing of the signal in order to derive desired characteristics.
A particular advantage of the invention arises in that since it is possible dramatically to remove motion artefact, the invention may be applied in situations where the light emitting means is not directly coupled to the living tissue by means of a clip or other attachment means, as in the prior art. In the prior art it was necessary to couple the light source or other radiation emitting means to a subject in order to reduce as far as possible motion artefact. However in the present invention, in appropriate circumstances, the light source and light receiving means may be mounted in a fixed installation with the patient or living tissue disposed adjacent for receiving radiation but not mechanically coupled to the installation. This introduces an important simplification in that it is not necessary to employ carefully designed clip-type arrangements, and permits use where contact probes fail, such as foetal monitoring, abulatory studies, neonatal monitoring and patient trauma conditions.

Whilst many light receiving diodes have logarithmic responses and will therefore naturally provide an output signal providing a logarithmic measure of input radiation, since such logarithmic responses are not strictly accurate in many cases, it is preferred to provide a light responsive diode with a linear response, and subsequently to provide a logarithmic amplifier which provides an accurate logarithmic measure of the input radiation. Naturally, a separate logarithmic amplifier will be provided in the respective channel for each wavelength signal. In a further aspect, the present invention provides apparatus for measuring or monitoring living tissue comprising emitting means for emitting electromagnetic radiation, means for receiving said radiation at the different wavelengths after it has been transmitted through or reflected within said tissue, the emitting means and receiving means being fixedly mounted with respect to one another, but there be no means provided for attaching the emitting means or receiving means to living tissue, the receiving means being arranged to provide at least first and second signals representing logarithmic measures of the received wavelength signals, and means for removing motion artefact from said signals and digital sampling means for sampling the motion artefact's removed signals, and means for processing the digital samples. In one embodiment the optical measuring device is a photoplethysmograph.

The device may be a pulse oximeter and include means for displaying the oxygen saturation of the patient's blood.
In this embodiment use may be made of the derived independence of an equalised pulsatile PPG signal from the tissue characteristics at the wavelength of the second (subtracted) signal. One of three optical wavelengths may be chosen as a control signal, such that there is a high contrast between the tissue characteristics at the signal and control wavelengths.

Brief Description of the Drawings

A preferred embodiment of the invention will now be described with reference to the accompanying drawings, on which:-

Figure 1 is a schematic block diagram of a known arrangement for reducing motion artefact;

Figure 2 is a schematic block diagram of the preferred embodiment of the invention; and Figure 3 and 4 are waveforms for explaining the application of the present invention.

Description of the Preferred Embodiment

The invention uses a general physical model (a more general model than applied in, say, the disclosure of WO 97/00041), which incorporates a means of ordering the dynamic components of the received signals with respect to both physiological and non-physiological dynamics. The ordering of dynamics implies a system which can derive the arterial oxygen saturation without the need to consider the static portions of the received signals, needing only the pulsatile components. In addition, this calculation is also independent of any tissue characteristics, both dynamic and static, at the control wavelength. Moreover, the oxygen saturation calculation contains only one unknown, which has a physical origin which can be bounded and estimated.

We examine the case when a number of light sources can be used in conjunction with a single receiver to generate one or more photoplethysmographic (PPG) signals. A PiN diode receiver is used, having a logarithmic response to the received light level. Although this non-linear response can lead to complexity in certain aspects of
measurement, however, it limits saturation and will therefore ultimately allow a greater degree of equalisation to be performed than with a linear receiver. A given PPG signal generated by the receiver may be described by

v(t,λ) = v0l n [l + iph (t,λ)z\ (1)

where the photocurrent ip„ is wavelength dependent through the diode responsivity, Z is a constant characteristic of the receiver, and v0 includes any gain being applied. Both Z and v0 should only be affected significantly by temperature. The photocurrent is linearly proportional to light intensity / at the receiver, which is modulated by pulsations and artefact as well as source characteristics and spatially dependent tissue characteristics. The received light intensity is modelled by

l(t,λ) = ∑ I l {t)[a,{t)+ f3 ι(t,λ) + y { )}

7=1 (2)

where the coupling coefficients depend on all the geometric, temporal and spectral properties of the source/receiver positioning, artefact, tissue dynamics and tissue optical properties. The coefficients a are interpreted as direct coupling between source and receiver and those labelled β, y correspond to light coupled via non-pulsatile and pulsatile tissue respectively. In many instruments the active light sources emit fixed power levels but here for the sake of generality they are allowed to vary. The ambient light is assumed to have been eliminated by an electronic multiplexing technique. Oximeters tends to use two or three (n=2,3) light sources and it may be assumed that, in the case of transmission mode, no light will be directly coupled to the receiver.

The nature of the coupling coefficients is now examined more closely. The values of a could also in principle depend on movement artefact, but only in the case of inappropriate sensor design. The values of β depend on tissue optical characteristics and movement artefact and include all effects not attributable to pulsatile signals. The
coupling of pulsatile signals modulated by movement artefact is represented by the γ coefficients which will normally be much smaller than the corresponding values of β. With these observations in mind we now make the following simplifying assumptions

, = const (3)

/?, W = /?, (ι + *, (0) I1 + >*( )] (4)

y (ή = rι[l + m(ή]p/(t) (5)

enabling equation (2) to be written in the form

The assumption (3) is valid for probes in which the optoelectronics have fixed relative positions or in which there is no direct coupling of light. Clearly this applies to the majority of conventional probes when operated normally. Equation (4) distinguishes movement artefact (m(t)) due to changes in probe coupling from artefact arising due to changes in non-pulsatile blood volume (b(t)). The constant of proportionality is free to change between given sources. This assumption is supported by our practical experience. Assumption (5) is justified by criteria similar to those for assumption (4) with the addition of independence between the pulsation dynamics and the movement artefact dynamics.

It is worth noting the ideal limit of negligible artefact, namely

yM pM (7)

for a constant light source, labelled by j, and receiver responsivity R; equation (7) can be approximated to first order in the size of the pulsatile signal as

There are a number of distinct elements of this model which can dominate the final signal and it will be important to analyse the detailed ordering of terms. Taking the case when the direct coupling of light is negligible, which should be a very good approximation for all transmission mode devices and suitably constructed reflection mode devices, and then using (1) and (6).

By further neglecting dark current compared with all the other sources of current we then obtain

;, (t,λ) = v„ (10)

It is interesting to observe that the assumptions (3) to (5) lead to a separation of probe coupling artefact into a single term not dependant on the light source, suggesting that its removal is trivial. It is important to realise that this degree of separation of movement artefact is only achieved because of the non-linear nature of the receiver.

Probe Coupling Artefact Equalisation

The distinctive nature of the present approach is that the effect of movement artefact has been modelled as a linear factor modulating the coupling coefficients of static and pulsatile tissue. This factor is assumed to scale with the unmodulated coupling coefficients but its functionality is unchanged. This assumption may be inaccurate for certain types of movement artefact (e.g. extreme degrees of movement), especially those which produce differential path length changes between sources and
receiver, mainly due to skewing of the probe geometry. Nevertheless the ensuant simplicity afforded justifies an investigation of this approximation. It is also worth noting that the approximation is more general than assuming additive signal artefact, an assumption which has received very serious attention (for example in WO 96/12435).

Removal of coupling artefact can be achieved by generating two independent measures of equation (10) and performing a subsequent subtraction, whence

7 (t,λ,b (i i)

Equation (11) indicates that an approach to equalisation based on the use of two light sources of contrasting tissue coupling characteristics will provide the most effective isolation of the pulsatile signal. Since oxy-, deoxy-haemoglobin and melanin are strong absorbers of blue light, significant contrast will be obtained between red (or infra-red) light and blue light. In that case, a linearisation of equation (1 1) yields

yll

= v„ n R,i,β Α + v,r r pλ ) (12)

Rkhβ )) " β,(λ)

where j labels the red (or infra-red) light and k labels the blue light. It is to be noted that the pulsatile component of the equalised signal is entirely independent of the blue light.

Consequences for Oximetry

The introduction of non-linearity at the front end amplifier has allowed us to develop a method for artefact equalisation but some rescaling of the pulsatile signals has been performed. It is now useful to explore the consequences on the determination of saturated oxygen. The Beer-Lambert formulation is usually used as a basis for the theoretical understanding of pulse oximetry and also benefits from simplicity. We use it here, mainly for the latter reason. The Beer-Lambert law couples path length r and effective absorbance μeJf together in a single definition of optical density, so that the total signal may be written as

s(t)= s0 exp(-μeff r) (13)

where s0 is a constant for given tissue type and probe placement. The effective absorbence may be interpreted as the combined effects of absorption and scattering. We now make an assumption on the separation of optical path into a small pulsatile part and a larger non-pulsatile dependent part

Meff ^ t^J^+ ^ - d) (14)

where (r-d) may still be dynamic, due for example to venous blood movement. The dynamic part of the equalised signal, from equation (12), together with (13) and (14) can be expressed as

VAC (15)

and changes in this component are found to be only dependent on the path through pulsatile issue. In the usual oximetry formulation we employ the wavelength dependency of μhhod to obtain

Mbhoλλ = (16) b oAλl)

where

from which the oxygen saturation is found to be

(18)

Equation (18) differs from the conventional pulse oximetry formulation only by the factor Tn which is a ratio of ratios and should be independent of any geometric coupling condition or individual tissue type. The difference in implementation between this formulation and conventional oximetry is that only the pulsatile signal is required for analysis (equation (16)).

To clearly illustrate the invention, Figures 1 and 2 show diagrammatic block diagrams of, respectively, a pulse oximeter device of the prior art and a device according to the invention. In Figure 1 an optical receiver 2 receives two light signals at different wavelengths (e.g. red visible and infrared). A demultiplexer 4 separates the two signals into two channels 6, 8 and passes them through pre-amplifiers 10. Each signal is passed through a parallel low-pass filter 12 and a high-pass/band-pass filter 14 in order to separate the dc and the ac components. Each component signal is then passed through a separate gain control 16, 17 since the dc and ac components are of very different orders of magnitude, before being multiplexed at 18 and passed into the DSP or microcontroller system 19 in which the comparisons can be made to produce an oxygen saturation measurement (Saθ2). Conventional devices may incorporate artefact reduction into the signal processing at this stage. In Figure 2, three signals are fed instead into logarithmic amplifiers as shown, and the difference signals between each of the outputs of the logarithmic amplifiers and
the control amplifier feed into a gain control. Note that in this design it is only necessary to sample the AC (pulsatile) signal components, since the static components play no part in the oxygen saturation calibration. This not only simplifies the electronics (note that only two channels are multiplexed to the microprocessor) but also reduces the complexity of the control algorithm.

Referring to Figure 2 in more detail, a light source 20 is fixedly mounted with an optical receiver 22 in a suitable mechanical installation, indicated as at 24. Living tissue schematic shown as at 26 is disposed between the light source and optical receiver 22, but is not mechanically affixed thereto. The light source emits three different wavelengths, normally red, infrared and a control source (e.g. blue) and the optical receiver has three separate light responsive diodes each responsive to their respective wavelength. The response of each diode is strictly linear in that a voltage or current output signal is provided which is linearly related to the intensity of radiation thereon. The output signals from the diodes are output on a single channel to a demultiplexer 28 which separates the three wavelength signals into respective channels 30, 32, 34. Each channel has a preamplifier 36 followed by a logarithmic amplifier 38 in order to give a logarithmic version of the signal. The signal in channel 32, representing the blue signal is used as a control channel and is subtracted from the infrared signal in channel 30 in a subtractor 40, and is subtracted from the red signal in 34 and subtractor 42. The outputs of the two subtractors are signals which represent equations (11) and (12), referred to previously. The outputs of the subtractors 40, 42 are applied to respective gain control units 44, 46, which gain controls units inherently remove dc components in the signal, i.e. the dc component represented in equation (12). Alternatively a high pass filter may be employed, with a cut off frequency of about 0.5 Hertz in order to remove the dc component. The outputs of the gain control unit are applied to a multiplexor 48, the output of the multiplexer being fed to an analogue to digital converter 50 which provides digital samples of the ac components which will be digital samples of the light passing through pulsatile tissue with motion artefact removed as in equation (12), to a digital system 52 for carrying out a required manipulations as set out above.

Artefact reduction may be incorporated into the DSP system, but the simple front-end analogue electronic arrangement automatically provides a signal
representative of the difference between the logarithms of the received signals, and therefore allows for compensation of the multiplicative element of the artefact. In addition, the logarithmic amplifiers handle the extremely wide dynamic range of the received signals without saturating.

It is to be noted that in the system of Figure 1 it is impossible to recover the multiplicative element of the signal since the ac and dc components have been separated by the time the signal is processed.

Implementation and testing

Artefact minimisation can be shown with the use of the simultaneous removal of ambient artefact and probe coupling motion artefact. Figures 3 illustrates a trace obtained without employing the artefact reduction of the present invention, and including a period of severe artefact. Figure 4 shows the same situation, but employing the artefact reduction of the invention: the artefact has been completely removed.

Because the present approach provides a good solution to the problem of probe coupling artefact, it allows the consideration of remote PPG and therefore remote pulse oximetry. Since a significant fraction of other artefact is actually induced by mechanical forces being coupled to the subject via the physical contact of the probe (secondary artefact) these will all be removed in a remote system. Furthermore the solution to probe coupling artefact will also reduce (but not remove) primary artefact caused by venous blood motions and any residual ambient artefact.

A further attractive property of this method is that the requisite technology is simple. It may be implemented by purely analogue electronics if necessary, and it is a strong candidate for miniaturisation.

Although the present invention has been described and illustrated purely in the context of optical measuring and monitoring devices, it is believed that it might also be implemented in alternative systems such as in the area of ultrasound devices where movement artefact can cause problems.

Claims

1. A method of monitoring living tissue comprising the steps of emitting electromagnetic radiation at said tissue at at least first and second different wavelengths, receiving the radiation at the different wavelengths after it has been transmitted through or reflected within said tissue, providing at least first and second signals which are a logarithmic measure of the received first and second radiation wavelengths and subtracting the second signal from the first signal, removing a DC component of the result of the subtraction and providing an AC component to digital sampling means, and processing the digital samples in order to provide a desired value representing a property of the tissue.

2. A method according to claim 1, including providing a third signal which is a logarithmic measure of a third radiation wavelength, subtracting the second signal from the third signal, removing a DC component of the result of the subtraction and providing an AC component to digital sampling means, and processing the digital samples in order to provide a desired value representing oxygen saturation of the tissue.

3. Apparatus for measuring or monitoring living tissue, comprising means for irradiating tissue with electromagnetic radiation at at least first and second different wavelengths, means for receiving the radiation at the different wavelengths after it has been transmitted through or reflected within said tissue for providing respective first and second wavelength signals which are a logarithmic measure of the received radiation signals, means for subtracting the second wavelength signal from the first wavelength signal, means for removing a DC component from the output of the subtracting means, digital sampling means for digitising said AC component, and processing means for processing the digital samples in order to derive a value representing a property of said tissue.

4. Apparatus for measuring or monitoring living tissue, comprising means for irradiating tissue with electromagnetic radiation at at least first and second different wavelengths, means for receiving the radiation at the different wavelengths after it has been transmitted through or reflected within said tissue, logarithmic amplifier means coupled to the receiving means for providing respective first and second wavelength signals which are a logarithmic measure of the received radiation signals, means for subtracting the second wavelength signal from the first wavelength signal, means for removing a DC component from the output of the subtracting means, and processing means for processing the digital samples in order to derive a value representing a property of said tissue.

5. Apparatus according to claim 3 or 4, wherein the removing means is provided by gain control means.

6. Apparatus according to claim 2 or 3. including means for providing a third wavelength signal which is a logarithmic measure of received radiation at a third wavelength, means for subtracting the second wavelength signal from the third wavelength signal, and means for removing a DC component from the output of the subtracting means

7. Apparatus for measuring or monitoring living tissue comprising emitting means for emitting electromagnetic radiation, means for receiving said radiation at the different wavelengths after it has been transmitted through or reflected within said tissue, the emitting means and receiving means being fixedly mounted with respect to one another, but there be no means provided for attaching the emitting means or receiving means to living tissue, the receiving means being arranged to provide at least first and second signals representing logarithmic measures of the received wavelength signals, and means for removing motion artefact from said signals and digital sampling means for sampling the motion artefact removed signals, and means for processing the digital samples.